I would say a 6-8 gallon tank with a betta a snail and 6 neon tetra.
Best way to do it is put all the old stuff in the new tank and rearrange it all, put the neon's in first, leave them for a while to get comfortable, then put the snail in then lastly, put your bettaboy in :) Good Luck

I wouldn't put neons in anything smaller than 10g but 15 would be ideal as a minimum neons being active little fishies despite their size they need plenty of swimming space. Also, it gives your betta plenty of room to get away from the neons and call his own. My betta has picked a corner of the tank which is "his" where the neons don't go and the corys are beginning to learn is Kyon's territory. He allows Altair to enter.

It's a hit or miss combination Lenimph. What works for one might not work for another. My tetras get along great with my betta because I had a lot of plant life and hiding spots. However, it is less likely to work if the tank is bare. If neons are kept in large enough groups (6+) it's more successful as the tetras will feel safer and will have less need to nip or harass your betta. I think when adding new fish to an aquarium with others you should do it when you know you'll have time to observe their behaviour incase of conflict.

My tank was heavily planted though at the time. Things seemed great before I left.

I didn't have much of a choice since my heater for my Betta's tank broke and I didn't want to leave him in a bowl since it was the winter and I knew my house would get cold. Neon tetra's are cute and all but there's a lot of other tetra variarities that fair better. But yeah it all depends on the personality of the betta really imo...

I wasn't rubbishing your oppinion :) It's working out for me but a lot of people here it hasn't. I love my little neons, much more than most of the other tetra types. I'm quite defensive of them too as they were my first tropical fish and I've always gone back to them. As with all combinations and when introducing new species to an established tank you should always have a back up plan if there's any aggression.

A 10 gallon tank is the minimum to keep betta with other fish, but if you are doing neons I would go up to a 15 or 20 gal (longer the better). Neons have a nippy reputation, but this can be fixed by keeping them in groups of 10 or more. Even in a group of 6 they will still be nippy.

Just as a general fishkeeping note, I don't recommend neon tetra at all. They need rather soft water to thrive and show their best colors. Due to inbreeding from production on a commercial scale, they are also rather weak and sickly. X-ray tetra (Pristella tetra), harlequin rasbora, and glowlight tetra make much better tankmates due to their adaptability to many different water supplies and their peacefulness. But all of these fish still need a minimum of 6. I recommend starting with 9 for better results that way in case you lose one, all hell won't break loose in the tank.

I would say 7.5 to 8 gallons for five neon tetras and a betta, providing that you have some live plants in there to help clean up, as well as a 10 gallon rated filter. You would need to make a baffle to decrease water flow.